In the field of clinical diagnosis, it is common to test certain ionic analytes using a potentiometric slide test element providing a differential potential between a reference liquid and the patient sample liquid, both of which are aqueous. Such an element features two substantially identical ion-selective electrodes held spaced apart in a frame, and an ion-junction bridge connecting them. The bridge is apertured so as to provide two liquid access apertures each aligned and in fluid contact with one of the electrodes. To ensure the two liquids will flow together within the bridge, the bridge is constructed to induce liquid flow away from the entrance aperture toward the other aperture. For example, the bridge can be a sheet of fibers, e.g., paper; on or embedded within the frame. An example of the first is shown in U.S. Pat. No. 4,053,381 and an example of the second is shown in U.S. Pat. No. 4,273,639.
Although such examples have worked admirably, they do have a minor disadvantage--liquid flow within the bridge tends to occur in all directions, rather than just from one aperture to the other. At best, this non-directional flow requires more than the minimum amount of liquid. At worst, liquid flowing in non-desired directions can lead to shorting.
As a result, mechanical means are preferably added to keep liquid flow from extending in the non-desired direction., namely, directions angled away from the straight-line direction between the apertures. An example of dams placed parallel to the straight-line direction to cut down on flow away from that direction, is shown in U.S. Pat. No. 4,556,474. Although such dams can reduce the amount of liquid required, they do further complicate the manufacturing process.
Thus, there has been a need, prior to the invention, for an ion-junction bridge that inherently favors flow directions that are only the desired flow directions, minimizing the volume of liquid needed.